Asynchronous Execution with cmd() Fails

[shangzhongq]

Issue: Asynchronous commands (e.g., after) executed via tcl.cmd() do not resolve correctly, returning undefined despite outputting to stdout.

Impact: Breaks asynchronous Tcl commands, limiting scripting capabilities.

Reproduction:

const Tcl = require('tcl').Tcl;
const tcl = new Tcl();
(async () => {
try {
const result = await tcl.cmd('puts [after 1000 "Async test"]');
console.log('Result:', result);
} catch (err) {
console.error('Error:', err.message);
}
})();

Observed Output: after#0\nResult: undefined

Expected Output: Async test\nResult: Async test

[uatuko]

await waits for a Promise, but cmd is expecting a callback and doesn't return a Promise. Until the code is updated to a Promise based API you can try the following workaround.

const { promisify } = require('node:util');

const Tcl = require('tcl').Tcl;
const tcl = new Tcl();

const cmd = promisify(tcl.cmd).bind(tcl);

(async () => {
	try {
		const result = await cmd('puts [after 1000 "Async test"]');
		console.log('Result:', result);
	} catch (err) {
		console.error('Error:', err.message);
	}
})();

[shangzhongq]

Hi, uatuko, Your suggestion is great. I was wondering how to register an async custom command with cmd? I tried your 'proc' suggestion in other reply,However, it seems cmd('proc myAsyncCmd {a b} {after 1000; return [expr {$a + $b}]}') does not work, I got 'invalid command name "myAsyncCmd"'Any insight is highly appreciated! -Zhong On Friday, September 12, 2025 at 02:04:12 AM PDT, Uditha Atukorala ***@***.***> wrote: uatuko left a comment (nukedzn/node-tcl#25) await waits for a Promise, but cmd is expecting a callback and doesn't return a Promise. Until the code is updated to a Promise based API you can try the following workaround. const { promisify } = require('node:util'); const Tcl = require('tcl').Tcl; const tcl = new Tcl(); const cmd = promisify(tcl.cmd).bind(tcl); (async () => { try { const result = await cmd('puts [after 1000 "Async test"]'); console.log('Result:', result); } catch (err) { console.error('Error:', err.message); } })(); — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.Message ID: ***@***.***>

[uatuko]

Unfortunately you can't register a node.js function to be available to be used with cmd(). As mentioned in #27 (comment), this is due to cmd() creating a new Tcl interpreter instance for every call so the context isn't preserved.

Currently you can only register node.js function to be available with cmdSync().

[shangzhongq]

Thanks for clarifying that. Is there any way to handle a true asynchronous node.js function? cmdSync can register any commands but does not wait for async function. Yahoo Mail: Search, Organize, Conquer On Sat, Sep 13, 2025 at 8:40 AM, Uditha ***@***.***> wrote: uatuko left a comment (nukedzn/node-tcl#25) Unfortunately you can't register a node.js function to be available to be used with cmd(). As mentioned in #27 (comment), this is due to cmd() creating a new Tcl interpreter instance for every call so the context isn't preserved. Currently you can only register node.js function to be available with cmdSync(). — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.Message ID: ***@***.***>

[shangzhongq]

This is a big limitation when I try to use an async node.js function within a Tcl for loop. e.g.,

for {set i 0} {$i < 3} {incr i} { myAsyncJSCmd $i }

custom async node.js function (command) inside a native Tcl command

[uatuko]

Hey @shangzhongq, I agree it would be better to be able to handle true async node.js functions. However this will probably need some fundamental architecture changes to run the Tcl Interpreter outside the main node.js event loop (and make it none blocking). There might be a quicker way to just block until node.js async function completes, but this needs more investigation.

Unfortunately I don't have capacity to look into this at the moment but if there's a compelling use-case I can look into prioritising.

What's your use-case?

[shangzhongq]

Hi, Uatuko,

I’m using your tcl Node.js package to integrate async JavaScript commands with Tcl, but I’ve encountered an issue where custom commands registered via tcl.registerCommand work standalone but fail in control structures like for loops. You mentioned that tcl.queue only shares context for async functions, which seems to cause this problem.

Setup:

• I’m using a patched Tcl interpreter (patched_tcl.js) with tcl.registerCommand to register async JS commands (e.g., myCmd) and tcl.queue for native Tcl commands (e.g., set, for).
• tcl.registerCommand stores commands in a JS object (this.commands) and attempts to register them in Tcl via cmdSync(proc ${name} {args} {return [${name} {*}$args]}).
• Standalone commands (e.g., myCmd test) work via tcl.eval, which checks this.commands.
• In for loops (e.g., for {set i 0} {$i < 3} {incr i} {myCmd $i}), tcl.queue evaluates the body, but myCmd fails with invalid command name "myCmd" because it’s not in Tcl’s native command table.

Test Case:
Attached is test_tcl_issue.js, which demonstrates the issue. It registers myCmd and tests it standalone (works) and in a for loop (fails).

test_tcl_issue.js

Request:

• Is there a way to register custom commands in Tcl’s native command table so they work in for loops via tcl.queue?
• Does the tcl package provide a method like interp.registerCommand for this?
• Any plans to extend tcl.queue to share the JS context for custom commands in control structures?

Thank you for your time!